now we're getting somewhere
i like the SSD explanation. It's the first time I have one so maybe as you say without any whirring of disks and fans other noise is more audible.
And the SSD wiki says: "SSDs have no moving parts and therefore are basically silent, although electric noise from the circuits may occur."
So maybe it is electric noise, indeed it does sound vaguely reminiscent of the crackle of a copper wire with a loose connection, when the current jumps an air gap